Genes and behaviour show ancient ties for man and mutt.
Men and dogs go way back. New studies suggest that dogs shared a hearth with early Stone Age humans and trotted beside them across the Bering Strait into the New World. Domestication may also have turned dogs into keen readers of human behaviour, researchers say.
Palaeolithic humans were probably the first to tame dogs (Canis familiaris) by breeding aggression out of wolves (Canis lupus) in East Asia around 15,000 years ago. So conclude Peter Savolainen and colleagues from comparing dog and wolf genes from around the world1. There are more unique genetic types of dog in China, Thailand, Cambodia, Tibet, Korea and Japan than in Europe, West Asia, Africa and Arctic America.
"It is more probable that all the modern types have come from East Asia to Europe than other way around," says Savolainen, an evolutionary biologist at the Royal Institute of Technology in Stockholm, Sweden. His group compared dog and wolf mitochondrial DNA (mDNA). This is inherited maternally, enabling the group to trace the dogs back to distinct female wolf lineages.
The dog genetic types fall on "different branches of the genetic tree of wolves", Savolainen says. The branches are far enough apart to represent individual female wolves, but close enough to represent a single wolf population. This, he says, hints that humans domesticated dogs several times in one part of East Asia.
Dogs from at least five domestic lineages probably accompanied humans across the land bridge from Siberia to Alaska and down into the Americas around 12,000-14,000 years ago, according to a similar genetic study by Jennifer Leonard and colleagues2.
These researchers looked at mDNA from ancient dogs from archaeological sites predating European settlements in Mexico, Bolivia, Peru and Alaska. These native American dogs were more closely related to Eurasian dogs and wolves than were American grey wolves.
People must have gained some "advantage by having this domestic animal at that early time", says Leonard, an evolutionary biologist now at the National Museum of Natural History in Washington DC. Dogs may have been used as sentinels, for transport, and for herding buffaloes or mastodon in hunts.
If dogs really were domesticated 15,000 years ago, says Leonard, then it's remarkable that they spread across three continents in only a few thousand years. At the time humans were still hunter-gatherers living in relatively isolated groups.
Exactly when dogs arrived on the scene is still uncertain. They could have shown up as early as 40,000 years ago, Savolainen's group calculates. But archaeological evidence for dogs is spotty.
Most dog fossils date only from about 7,000 years ago, says Juliet Clutton-Brock, an expert on the history of domestic mammals working at the Zoological Society of London. "So even 15,000 years ago is too early."
However long is the shared history between us and our four-legged friends, it seems to have led to a deep understanding. Dogs pick up human hints about hidden food better than chimpanzees or human-reared wolves, Brian Hare and colleagues have shown3. Puppies of all ages excel at following a human's gaze or pointing to food, even if the animals have had little experience of humans.
Chimps are our closest relatives, says Hare, a biological anthropologist at Harvard University in Cambridge, Massachusetts, "but they cooperate and communicate with humans very poorly". Distantly related dogs, on the other hand, having evolved alongside humans, seem to have converged on some of our thought processes, he says.
Whether social sense arose in dogs because it was advantageous for domestication or whether it was simply a by-product, like traits such as floppy ears, remains to be investigated.
Savolainen, P. et al. Genetic evidence for an East Asian origin of domestic dogs. Science, 298, 1610 - 1613, (2002).
Leonard, J.A. et al. Ancient DNA evidence for Old World origin of New World dogs. Science, 298, 1613 - 1616, (2002).
Hare, B. et al. The domestication of social cognition in dogs. Science, 298, 1634 - 1636, (2002).
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Powell, K. Stone Age man kept a dog. Nature (2002). https://doi.org/10.1038/news021118-12